The present invention relates generally to wheel servicing machines such as wheel balancers and tire changing machines. More particularly, the present invention relates to devices and methods for identifying material interfaces on or near a tire using active sonar.
Balancing machines are well-known in the art for measuring rotodynamic imbalance in a rotating body, such as a vehicle wheel. Wheel balancing machines, or wheel balancers, typically include a rotatable shaft directly or indirectly coupled to a motor or drive mechanism. A vehicle wheel can be releasably mounted on the shaft and rotated. Various forces associated with static or dynamic imbalance properties of the wheel are measured using one or more force transducers linked to the shaft. The measured imbalance forces can be correlated with the wheel dimensions in a computer-based algorithm to determine the locations of any asymmetric mass distribution present in the wheel. Such unequal mass distribution will cause the wheel to be “unbalanced.” Based on the measured forces, corrective weights may then be applied to the rim to balance the wheel.
Three primary wheel assembly dimensions are needed to determine where the corrective wheel weights should be positioned. These dimensions generally include the “A” distance between the wheel rim and the right force transducer, the “D” distance corresponding to the diameter of the wheel rim, and the “W” distance corresponding to the axial width of the wheel rim. Using these three dimensions together with the imbalance forces measured by the force transducers during a test spin of the wheel, a pre-determined wheel balancing algorithm can calculate and identify to an operator the optimal locations for application of corrective weights to balance the wheel.
Conventional wheel balancing machines can include one or more devices for manually or automatically determining the A and D dimensions of the wheel assembly. For example, U.S. Pat. No. 7,882,739 teaches a wheel balancer including a data acquisition arm, or A&D arm, for measuring the A and D dimensions. However, the width dimension W is more difficult to measure because of fluctuations in the local width of the wheel assembly. For example, many styles of modern tires include variations in local width corresponding to different radial and/or angular locations along the outer surfaces of the tire. It is important that the W dimension used in the balancing algorithm be the W dimension corresponding to the width of the wheel rim at the outermost radial wheel rim location. The W dimension should not correspond to a local width on the tire. When an improper width dimension W is used in the balancing algorithm, an improper corrective weight location may be calculated, and the wheel will not be properly balanced.
During a conventional wheel balancing procedure, a machine operator may manually measure the width dimension W using a pair of calipers. However, such conventional methods of width determination are often time-consuming and may lead to error or miscalculation. To overcome this problem, others have developed improved methods and devices for automatically determining the width dimension W. For example, U.S. Pat. No. 5,189,912 teaches an ultrasonic wheel measuring apparatus and wheel balancer that uses an acoustic, or sonar, signal to measure the distance between a reference plane and the local wheel assembly surface. That measured distance can be used to calculate width measurement W. Such conventional acoustic devices for measuring wheel assembly width rely on local differences in distance between the acoustic transducer and the wheel assembly surface to calculate wheel assembly width. As the sonar signal articulates past the wheel profile, the time of flight of the composite incident and reflected acoustic wave is used to determine a local distance between the wheel assembly surface and the transducer. A queue of distance samples is then compiled, with each sample corresponding to a different radial location on the wheel assembly surface. A processor then attempts to identify the tire/rim transition location based on a change or pattern in the measured distance sample queue. The measured distance sample corresponding to that radial position is then used to calculate the width dimension W.
Conventional sonar distance measurement devices for determining width dimension W on a wheel balancing machine based solely on an acoustic wave time of flight calculations can generate erroneous determinations of tire/rim interface location in many applications. For example, modern tires having a contoured or textured outer side wall may cause variations in sonar time of flight signature that are similar to the predetermined signature identified for a tire/rim interface, but correspond only to changes in the structure of the tire. When a corresponding balancing operation is performed based on the erroneous width measurement W, the operator may be instructed to apply weights at improper locations, resulting in a wheel that is improperly balanced.
What is needed, then, are improvements in the devices and methods for determining wheel assembly dimensions using acoustic signal processing.